NTC THERMISTORS: TYPE CL
NTC DISCS FOR INRUSH CURRENT LIMITING
DESCRIPTION:
Disc thermistor with uninsulated lead-wires.
FEATURES:
• Low cost solid state device for inrush current
suppression
• Excellent mechanical strength
• Wide operating temperature range: -50°C to 175°C
• Suitable for PCB mounting
• Available as a standard with kinked leads and on
tape and reel to EIA RS-468A for automatic insertion
TYPE
Fig. 1
CL-11
CL-21
CL-30
CL-40
CL-50
CL-60
CL-70
CL-80
CL-90
CL-101
CL-110
CL-120
CL-130
CL-140
CL-150
CL-160
CL-170
CL-180
CL-190
CL-200
CL-210
Max*
Steady
Res @ State
Disc
25°C Current Dia.
±25% AMPS (Max)
(ohms) (RMS)
(in.)
0.7
1.3
2.5
5
7
10
16
47
120
0.5
10
10
50
50
5
5
16
16
25
25
30
12
8
8
6
5
5
4
3
2
16
3.2
1.7
1.6
1.1
4.7
2.8
2.7
1.7
2.4
1.7
1.5
0.77
0.55
0.77
0.77
0.77
0.77
0.77
0.77
0.93
0.93
0.40
0.40
0.45
0.45
0.55
0.55
0.55
0.55
0.55
0.55
0.40
Lead
Disc
Thick. Spacing
(Ref.)
(Max)
(in.)
(in.)
0.22
0.21
0.22
0.22
0.26
0.22
0.22
0.22
0.22
0.22
0.17
0.17
0.17
0.17
0.18
0.18
0.18
0.18
0.18
0.18
0.20
0.328
0.328
0.328
0.328
0.328
0.328
0.328
0.328
0.328
0.328
0.250
0.250
0.250
0.250
0.328
0.328
0.328
0.328
0.328
0.328
0.250
Lead
Dia.
AWG
18
18
18
18
18
18
18
18
18
18
24
24
24
24
22
22
22
22
22
22
24
Cx (max)**
µFarads
Equation constants for
resistance under load ***
@120
VAC
@240
VAC
X
2700
800
6000
5200
5000
5000
5000
5000
5000
4000
600
600
600
600
1600
1600
1600
1600
800
800
600
600
200
1500
1300
1250
1250
1250
1250
1250
1000
150
150
150
150
400
400
400
400
200
200
150
0.50
0.60
0.81
1.09
1.28
1.45
1.55
2.03
3.04
0.44
0.83
0.61
1.45
1.01
0.81
0.60
1.18
0.92
1.33
0.95
1.02
Approx. Res. Under Load at %
Max. Rated Current
Y
Current
Range
Min. I / Max. I
25%
50%
75%
100%
-1.18
-1.25
-1.25
-1.27
-1.27
-1.30
-1.26
-1.29
-1.36
-1.12
-1.29
-1.09
-1.38
-1.28
-1.26
-1.05
-1.28
-1.18
-1.34
-1.24
-1.35
4.0≤ 1≤ 12
3.0≤ 1≤ 8.0
2.5≤ 1≤ 8.0
1.5≤ 1≤ 6.0
1.5≤ 1≤ 5.0
1.2≤ 1≤ 5.0
1.0≤ 1≤ 4.0
0.5≤ 1≤ 3.0
0.5≤ 1≤ 2.0
4.0≤ 1≤ 16
0.7≤ 1≤ 3.2
0.4≤ 1≤ 1.7
0.4≤ 1≤ 1.6
0.2≤ 1≤ 1.1
1.0≤ 1≤ 4.7
0.8≤ 1≤ 2.8
0.5≤ 1≤ 2.7
0.4≤ 1≤ 1.7
0.5≤ 1≤ 2.4
0.4≤ 1≤ 1.7
0.3≤ 1≤ 1.5
14
.25
.34
.65
.96
1.09
1.55
2.94
7.80
.09
1.10
1.55
5.13
5.27
.66
.87
1.95
2.52
2.63
2.74
3.83
.06
.09
.14
.27
.40
.44
.65
1.20
3.04
.04
.45
.73
1.97
2.17
.27
.42
.80
1.11
1.04
1.18
1.50
.04
.06
.09
.16
.24
.26
.39
.71
1.75
.03
.27
.46
1.13
1.28
.16
.27
.48
.69
.60
.70
.87
.02
.04
.06
.11
.16
.18
.27
.49
1.18
.02
.18
.34
.75
.89
.11
.20
.33
.49
.41
.49
.60
OPTIONS:
DATA:
• For kinked leads, add suffix “A”
• For tape and reel, add suffix “B”
• For tape and reel, add suffix “AB”
• Other tolerances in the range 0.7Ω to 120Ω
• Other tolerances, tolerances at other temperatures
• Alternative lead lengths, lead materials, insulations
*maximum rating at 25ºC or
BOWTHORPE THERMOMETRICS
Crown Industrial Estate, Priorswood Road
Taunton, Somerset TA2 8QY UK
Tel +44 (0) 1823 335200
Fax +44 (0) 1823 332637
Iderated =
Diss.
Time
Const. Const.
(mW/°C) (sec.)
25
15
25
25
25
25
25
25
30
30
8
4
8
4
15
9
15
9
15
9
8
100
60
100
100
120
100
100
100
120
120
30
90
30
90
110
130
110
130
110
130
30
√(1.1425–0.0057 x TA) x Imax @ 25°C
for ambient temperatures other than 25ºC.
**maximum ratings
***R0=X1Y where X and Y are found in the table above
THERMOMETRICS, INC.
808 US Highway 1
Edison, New Jersey 08817-4695 USA
Tel +1 (732) 287 2870
Fax +1 (732) 287 8847
KEYSTONE THERMOMETRICS CORPORATION
967 Windfall Road
St. Marys, Pennsylvania 15857-3397 USA
Tel +1 (814) 834 9140
Fax +1 (814) 781 7969
NTC THERMISTORS: TYPE CL
NTC DISCS FOR INRUSH CURRENT LIMITING
RI
-tº
TYPICAL POWER SUPPLY CIRCUIT
INRUSH CURRENT LIMITERS IN
SWITCHING POWER SUPPLIES
•
•
The problem of current surges in switch-mode power supplies
is caused by the large filter capacitors used to smooth the ripple in the rectified 60Hz current prior to being chopped at a
high frequency. The diagram above illustrates a circuit commonly used in switching power supplies.
•
•
•
In the circuit above the maximum current at turn-on is the
peak line voltage divided by the value of R; for 120v, it is
approximately 120 x 公2/RI. Ideally, during turn-on RI should be
very large, and after the supply is operating, should be
reduced to zero. The NTC thermistor is ideally suited for this
application. It limits surge current by functioning as a power
resistor which drops from a high cold resistance to a low hot
resistance when heated by the current flowing through it.
Some of the factors to consider when designing NTC thermistor as an inrush current limiter are:
BOWTHORPE THERMOMETRICS
Crown Industrial Estate, Priorswood Road
Taunton, Somerset TA2 8QY UK
Tel +44 (0) 1823 335200
Fax +44 (0) 1823 332637
Maximum permissible surge current at turn-on
Matching the thermistor to the size of the filter
capacitors
Maximum value of steady state current
Maximum ambient temperature
Expected life of the power supply
Maximum Surge Current
The main purpose of limiting inrush current is to prevent
components in series with the input to the DC/DC convertor
from being damaged. Typically, inrush protection prevents
nuisance blowing of fuses or breakers as well as welding of
switch contacts. Since most thermistor materials are very
nearly ohmic at any given temperature, the minimum no-load
resistance of the thermistor is calculated by dividing the peak
input voltage by the maximum permissible surge current in
the power supply (Vpeak/Imax surge).
THERMOMETRICS, INC.
808 US Highway 1
Edison, New Jersey 08817-4695 USA
Tel +1 (732) 287 2870
Fax +1 (732) 287 8847
KEYSTONE THERMOMETRICS CORPORATION
967 Windfall Road
St. Marys, Pennsylvania 15857-3397 USA
Tel +1 (814) 834 9140
Fax +1 (814) 781 7969
Energy Surge at Turn-On
At the moment the circuit is energized, the filter caps in a
switcher appear like a short circuit which, in a relatively short
period of time, will store an amount of energy equal to
1⁄2CV2. All of the charge that the filter capacitors store must
flow through the thermistor. The net effect of this large current surge is to increase the temperature of the thermistor
very rapidly during the period the capacitors are charging. The
amount of energy generated in the thermistor during this
capacitor-charging period is dependent on the voltage waveform of the source charging the capacitors. However, a good
approximation for the energy generated by the thermistor during this period is 1⁄2CV2 (energy stored in the filter capacitor).
The ability of the NTC thermistor to handle this energy surge
is largely a function of the mass of the device. This logic can
be seen in the energy balance equation for a thermistor being
self-heated:
As more current flows through the device, its steady-state
operating temperature will increase and its resistance will
decrease. The maximum current rating correlates to a
maximum allowable temperature.
In the table of standard inrush current limiters is a list of values for resistance under load for each unit, as well as a recommended maximum steady-state current. These ratings
are based upon standard PC board heat sinking, with no air
flow, at an ambient temperature of 25°C. However, most
power supplies have some air flow, which further enhances
the safety margin that is already built into the maximum current rating. To derate the maximum steady state current for
operation at elevated ambient temperatures, use the following
equation:
Iderated =
√(1.1425–0.0057 x TA) x Imax @ 25°C
Input Energy = Energy Stored
+ Energy Dissipated
o r in differential form:
where:
P =
t =
H =
T =
δ =
TA =
Pdt = HdT + δ(T – TA)dt
Power generated in the NTC
Time
Heat capacity of the thermistor
Temperature of the thermistor body
Dissipation constant
Ambient temperature
During the short time that the capacitors are charging (usually less than 0.1 second), very little energy is dissipated.
Most of the input energy is stored as heat in the thermistor
body.
In the table of standard inrush limiters there is listed a recommended value of maximum capacitance at 120 volts and
240 volts. This rating is not intended to define the absolute
capabilities of the thermistors; instead, it is an experimentally determined value beyond which there may be some reduction in the life of the inrush current limiter.
Maximum Steady-State Current
The maximum steady-state current rating of a thermistor is
mainly determined by the acceptable life of the final products
for which the thermistor becomes a component. In the
steady-state condition, the energy balance in the differential
equation already given reduces to the following heat balance
formula:
Power = I2R = δ(T – TA)
BOWTHORPE THERMOMETRICS
Crown Industrial Estate, Priorswood Road
Taunton, Somerset TA2 8QY UK
Tel +44 (0) 1823 335200
Fax +44 (0) 1823 332637
Inrush Current Limiters
THERMOMETRICS, INC.
808 US Highway 1
Edison, New Jersey 08817-4695 USA
Tel +1 (732) 287 2870
Fax +1 (732) 287 8847
KEYSTONE THERMOMETRICS CORPORATION
967 Windfall Road
St. Marys, Pennsylvania 15857-3397 USA
Tel +1 (814) 834 9140
Fax +1 (814) 781 7969